Because of the Seasons system, people have a lot more coins to spend (end of season bonuses make playing games that much more profitable). Greater amounts of disposable income = more people willing and able to pay more for players, and also more coins for people to play the market with to create additional wealth.

Also, the FUTHEAD stats you are seeing right now might only be reflecting the prices as of the latter stages of FUT 12, when prices tanked in the shadow of the FUT 13 launch.

Well people are downvoting him because he's missed the biggest reason, i.e., inflation with respect to last year. There's simply more coins in the market, which always drives up prices; cardweight and real world performances only affect certain players, which is not what OP asked about.